Trace element partitioning in the Fe-S-C system and its implications for planetary differentiation and the thermal history of ureilites

Element partitioning in metal-light element systems is important to our understanding of planetary differentiation processes. In this study, solid-metal/liquid-sulfide, liquid-metal/liquid-sulfide and solid-metal/troilite partition coefficients (D) were determined for 18 elements (Ag, As, Au, Co, Cr, Cu, Ge, Ir, Ni, Os, Pd, Pt, Mo, Mn, Re, Ru, Se and W) in the graphite-saturated Fe-S-C system at 1 atm. Compared at the same liquid S concentration, the solid/liquid partition coefficients are similar to those in the Fe-S system, but there are systematic differences that appear to be related to interactions with carbon dissolved in the solid metal. Elements previously shown to be “anthracophile” generally have larger solid/liquid partition coefficients in the Fe-S-C system, whereas those that are not have similar or smaller partition coefficients in the Fe-S-C system. The partitioning of trace elements between C-rich and S-rich liquids is, in most cases, broadly similar to the partitioning between solid metal and S-rich liquid. The highly siderophile elements Os, Re, Ir and W are partitioned strongly into the C-rich liquid, with D ? 100. The partition coefficients for Pt, Ge and W decrease significantly at the transition to liquid immiscibility, while the partition coefficient for Mo increases sharply. The bulk siderophile element patterns of ureilite meteorities appear to be better explained by separation of S-rich liquid from residual C-rich metallic liquid at temperatures above the silicate solidus, rather than by separation of S-rich liquid from residual solid metal at lower temperatures.     

Melting of enriched mantle beneath Pitcairn seamounts: Unusual U–Th–Ra systematics provide insights into melt extraction processes

U-series systematics as well as Sr isotopes were measured on young seamount lavas from the Pitcairn hotspot collected during the Polynaut cruise. The combined U-series and Sr isotope data reveal typical mixing relationships between two endmembers. One typical ‘plume’ endmember with radiogenic ⁸⁷Sr/⁸⁶Sr and relatively low ²³⁰Th/²³⁸U and a ‘lithosphere’ endmember with less radiogenic ⁸⁷Sr/⁸⁶Sr and relatively larger ²³⁰Th/²³⁸U. Remarkably, all the lavas, except for a few arguably older samples, are characterized by ²²⁶Ra deficits relative to ²³⁰Th. On the basis of water content and trace element systematics, we argue that this is due to melting in the presence of phlogopite, which is only stable at lithosphere temperatures. A melting model including the diffusive exchange of elements among phlogopite, garnet and melt is used to constrain melting conditions of the lithosphere. These unusual ²²⁶Ra–²³⁰Th signatures can be explained by relatively slow melting rates at low matrix porosity. Our model also demonstrates that the effective partitioning behavior is dependent on the melting rate. A simple thermal model for lithosphere heating and melting is in good agreement with predicted melting rates.     

The Comparison of δ<Superscript>13</Superscript>C Values of a Deposit- and a Suspension-Feeder Bio-Indicates Benthic vs. Pelagic Couplings and Trophic Status in Contrasted Coastal Ecosystems

δ¹³C and δ¹⁵N values of two generalists primary consumers, a strict deposit-feeder polychaete (Arenicola marina), and a strict suspension-feeder bivalve (Crassostrea gigas), were investigated to typify the trophic functioning of two contrasted marine coastal ecosystems (eutrophic and mesotrophic, east and west Cotentin peninsula, respectively, English Channel, Normandy, France). On average, δ¹³C and δ¹⁵N values of lugworms mirrored those of sediment organic matter (SOM), whereas δ¹³C and δ¹⁵N of oysters mirrored those of suspended particulate organic matter (SPOM). δ¹³C values of the two species displayed significant differences on the west coast (mesotrophic) contrary to the east coast (eutrophic; significant interactions). δ¹⁵N values differed only between sites and not between species. Diet of A. marina relied exclusively on microphytobenthos (MPB) and detritus of macroalgae (ULV) on the mesotrophic coast, whereas diet of C. gigas relied mainly on SPOM. Conversely, on the eutrophic ecosystem (the east coast), both species displayed the same diet, which was a mixture of pelagic sources (SPOM), benthic sources (MPB and ULV) and to a lesser extent riverine particulate organic matter (rPOM). These results were explained by the intensity of benthic vs. pelagic couplings (i.e. benthic-pelagic and pelagic-benthic) which differed in the two ecosystems. Low trophic coupling occurred on the mesotrophic (west) coast, whereas benthic-pelagic (SOM resuspension) and pelagic-benthic (settling of SPOM such as phytoplankton blooms) couplings were typified on the eutrophic (east) coast. This higher particulate organic matter (POM) pelagic-benthic coupling on the east coast was probably enhanced by nutrient enrichment caused by eutrophication. Comparison of δ¹³C ratios of both the strict deposit-feeder (e.g. A. marina) and the strict suspension-feeder (e.g. C. gigas) was then proposed as a bio-indicator of the trophic status and of POM benthic vs. pelagic couplings of soft-bottom coastal ecosystems.     

Improving the way we think about projecting future energy use and emissions of carbon dioxide

A variety of decision makers need projections of future energy demand, CO₂ emissions and similar factors that extend many decades into the future. The past performance of such projections has been systematically overconfident. Analysts have often used scenarios based on detailed story lines that spell out “plausible alternative futures” as a central tool for evaluating uncertainty. No probabilities are typically assigned to such scenarios. We argue that this practice is often ineffective. Rather than expanding people’s judgment about the range of uncertainty about the future, scenario-based analysis is more likely to lead to systematic overconfidence, to an underestimate of the range of possible future outcomes. We review relevant findings from the literature on human judgment under uncertainty and discuss their relevance to the task of making probabilistic projections. The more detail that one adds to the story line of a scenario, the more probable it will appear to most people, and the greater the difficulty they likely will have in imagining other, equally or more likely, ways in which the same outcome could be reached. We suggest that scenario based approaches make analysts particularly prone to such cognitive biases, and then outline a strategy by which improved projections, tailored to the needs of specific decision makers, might be developed.     

A one hundred and seventeen year coastal water temperature record from Woods Hole,Massachusetts

We have compiled what we believe is the longest coherent coastal sea surface temperature record in North America. Near-surface water temperature measurements have been made almost daily at Great Harbon, Woods Hole, Massachusetts, since 1886 with remarkably few gaps. The record shows that there was no significant trend in water temperature at this site for the first 60 yr of observation. There was some cooling during the 1960s that was followed by a significant warming from 1970–2002 at a rate of 0.04°C yr^?1. During the 1990s annual mean temperatures averaged approximately 1.2°C warmer than they had been on average between 1890 and 1970; winter (December, January, and February) temperatures were 1.7°C warmer and summer (June, July, and August) temperatures were 1.0°C warmer. There has not been a statistically significant decrease in the annual number of winter days below 1°C or an increase in the annual number of winter days above 5°C. The number of summer days each year with water temperature above 21°C has not increased significantly. The dates of first observations of 10°C and 20°C water in the spring have not changed sufficiently to be statistically significant. There is a weak positive correlation between annual and winter water temperature and the annual and winter North Atlantic Oscillation index, respectively, during the period of record.     

Econometric forecasting: A brief survey of current and future techniques

Amongst the techniques discussed are (a) for univariate series, generalisations of Box-Jenkins auto-regressive-moving average models including intervention analysis, non-linear models, and models with time varying parameters; (b) for single output, multiple input situations, causality testing, and diagnostic testing of alternative specifications using Lagrange multiplier tests; and (c) for multivariate cases, vector autoregressive models, order-selection criteria, Bayesian priors, and factor models. Co-integration and error-correction models are also introduced.Some of these techniques are illustrated using a forecast comparison exercise concerning forecasts of monthly electricity demand per customer.     

Fractional melting and smelting on the ureilite parent body

We investigate petrologic and physical aspects of melt extraction on the parent asteroid of the ureilite meteorites (UPB). We first develop a petrologic model for simultaneous melting and smelting (reduction of FeO by C) at various depths. For a model starting composition, determined from petrologic constraints to have been CV-like except for elevated Ca/Al (2.5 × CI), we determine (1) degree of melting, (2) the evolution of mg, (3) production of CO + CO₂ gas and (4) the evolution of mineralogy in the residue as a function of temperature and pressure. We then use these relationships to examine implications of fractional vs. batch melt extraction.In the shallowest source regions (∼30 bars), melting and smelting begin simultaneously at ∼1050 °C, so that mg and the abundance of low-Ca pyroxene (initially pigeonite, ultimately pigeonite + orthopyroxene) begin to increase immediately. However, in the deepest source regions (∼100 bars), smelting does not begin until ∼1200 °C, so that mg begins to increase and low-Ca pyroxene (pigeonite) appears only after ∼21% melting. The final residues in these two cases, obtained just after the demise of augite, match the end-members of the ureilite mg range (∼94-76) in pyroxene abundance and type. In all source regions, production of CO + CO₂ by smelting varies over the course of melting. The onset of smelting results in a burst of gas production and very high incremental gas/melt ratios (up to ∼2.5 by mass); after a few % (s)melting, however, these values drastically decline (to <0.05 in the final increments).Physical modelling based on these relationships indicates that melts would begin to migrate upwards after only ∼1-2% melting, and thereafter would migrate continuously (fractionally) and rapidly (reaching the surface in < a year) in a network of veins/dikes. All melts produced during the smelting stage in each source region have gas contents sufficient to cause them to erupt explosively and be lost. However, since in all but the shallowest source regions part of the melting sequence occurs without smelting, fractional melting implies that a significant fraction of UPB melts may have erupted more placidly to form a thin crust (∼3.3 km thick for a 100 km radius body).Our calculations suggest that melt extraction was so rapid that equilibrium trace element partitioning may not have been attained. We present a model for disequilibrium fractional melting (in which REE partitioning is limited by diffusion) on the UPB, and demonstrate that it produces a good match to the ureilite data. The disequilibrium model may also apply to trace siderophile elements, and might help explain the “overabundance” of these elements in ureilites relative to predictions from the smelting model.Our results suggest that melt extraction on the UPB was a rapid, fractional process, which can explain the preservation of a primitive oxygen isotopic signature on the UPB.     

Molecular dynamics investigation of viscosity, chemical diffusivities and partial molar volumes of liquids along the MgO-SiO2 join as functions of pressure

Molecular dynamics simulations are carried out to systematically address the effects of composition and pressure on melts along the MgO-SiO₂ join and elucidate the role of structural modifier content on silicate melt properties. The MgO-SiO₂ system shows non-ideal mixing with a negative excess volume of mixing at low pressures, but the mixing becomes closer to ideal at higher pressures. At atmospheric pressure, the viscosities and diffusivities vary by more than 3 orders of magnitude as the composition is varied along this join, with the low SiO₂ melts characterized by lower viscosities and higher diffusivities; these results are in quantitative agreement with experimental results for the dependence of viscosity and diffusivity on structural modifier content in a wide range of silicate systems. The transport properties of melts in this system converge at higher pressures; at pressures greater than ∼15 GPa the viscosity and diffusivities vary by less than an order of magnitude across the entire range of compositions. The relevance of equations that relate the viscosity and diffusivity is also addressed.